Generally, a transmission provides controlled application of engine power by conversion of speed and torque from a power source, such as for example an internal combustion engine. The hydraulic system may provide for actuation of friction elements in the vehicle transmission for coupling the transmission input to the geartrain to transmit engine power to the wheels of the vehicle. For example a clutch module in a dual clutch transmission (DCT) typically comprises two friction clutches for coupling the engine via a geartrain to the wheels by actuation of these clutches via said hydraulic system. In a variant one or more clutches can be made by using a powersplit mechanism with three rotational members where one member is connected to the input, one member is connected to the output and the third member can be connected to the transmission housing by means of actuation of a friction brake. Multiple configurations of these friction elements (clutch, brake) can be made resulting in various layouts of multi-friction transmissions. A transmission system of this type is know from e.g. US2013184119.
The brake and/or clutch elements can generate a considerable heat and the hydraulic system may also provide cooling fluid to each of the clutches and/or brakes of the transmission.
In a multiple friction transmission, such as for example a dual clutch transmission (DCT), a dual wet clutch may be oil cooled. Typically, the electrohydraulic control of the dual clutch transmission provides significant improved efficiency and performance, while maintaining the full shift comfort of traditional step automatics. A precise and fast clutch control can be made possible by direct acting solenoids, which are electromechanically operated valves.
Fundamentally, a DCT can be of the wet clutch or the dry clutch design. A wet clutch design is preferably used for higher torque engines, whereas the dry clutch design is generally suited for smaller torque engines. Although the dry clutch variants of a DCT may be limited in torque generation, compared to their wet clutch counterparts, the dry clutch variants may offer an improved fuel efficiency, mainly due to the cooling and lubrication. The wet clutch requires pumping transmission fluid in the clutch housing, which results in losses. Therefore, additionally, the cooling system in a multiple friction transmission may play an important role for the overall efficiency of the transmission.
A DCT layout is equivalent of having two transmissions in one housing which can be shifted and clutched independently, i.e. one power transmission assembly on each of the two input shafts together driving one output shaft, to enable uninterrupted gear shifting transmission in an automatic transmission form, while keeping high mechanical efficiency compared to a manual transmission.
The pump pressure of the clutch and/or brake actuation line in the hydraulic system determines whether the clutch element and/or brake element is actuated or not. When the actuation pressure in the clutch actuation line is low, then the clutch is disengaged. Commonly, a normally closed (NC) solenoid valve is arranged in the clutch actuation line, so that in the event that the first pressure circuit needs to be depressurized, for example due to a failure created by an electric malfunction, the valve will automatically close to prevent loss of control. However, if the valve fails in the open position, pressure in the clutch actuation line may stay high, which is undesirable. It is desirable that this state, wherein the clutch remains under pressure by which the drive of the engine cannot be disengaged, is avoided. Thus, if the hydraulic pressure in the clutch and/or brake actuation line is unexpectedly too high, for any reason whatsoever, a loss of user control over the vehicle may be induced. In the prior art, this problem is commonly addressed by providing two NC solenoid valves in the actuation lines of the hydraulic circuit, which are connected hydraulically one behind the other in series. In case of failure of one of the two said valves in an open state, the other valve can still be used to control the pressure in the clutch and/or brake actuation line.
However, the above-mentioned technical solution for providing a redundant fail safe is typically expensive and economically not attractive, since the arrangement of solenoid valves, comprising electronic components, is significantly more expensive than spool valves, i.e. valves controlled by a terminal fluid pressure. Furthermore, a spool valve is typically more robust and has a longer lifetime, compared to a solenoid valve.
Publication US2007/0107421A1 discloses a hydraulic circuit for an engine driven vehicle, comprising a higher pressure circuit and a lower pressure circuit. The hydraulic system controls fluid communication with an infinitely variable transmission (IVT) which includes higher pressure hydrostatic unit controls and lower pressure IVT hydraulic control clutches. When the demand of the higher pressure controls are satisfied, a relief valve opens and supplies fluid from the first high pressure supply line to the second lower pressure supply line. In this way, the low pressure pump can be minimally sized so as to supply only the normal requirements of clutch unit. However, a pressure-build up in a friction element actuation line, such as a clutch actuation line and/or a brake actuation line, connected to the higher pressure circuit, may result in a loss of control of the engine driven vehicle.
An insufficient cooling may lead to shortened component life and ultimately failure of the clutch assemblies within the multiple friction transmission. Moreover, insufficient cooling can be responsible for rapid degradation of the physical properties of the transmission fluid which may result a failure of other components within the multiple friction transmission.
Typically the clutch assemblies are cooled by transmission fluid, in a generally uncontrolled fashion, in order to provide sufficient cooling for the excessive heat generated in the multiple friction transmission. However, this cooling strategy typically goes hand-in-hand with losses in efficiency by excessively flooding of the clutch assemblies with fluid to provide sufficient heat reduction.
Also, high loading conditions may result in rapid generation of excessive heat. Conventional heat strategies are typically not appropriate or adequate to efficiently dissipate said rapid heat build-up. Therefore, excessive demands is put on the pump for providing the demanded fluid in these cases.
In the prior art, conventional cooling approaches of a multiple friction transmission typically use a single hydraulic cooling circuit to supply cooling fluid from the cooler device to the clutches. The cooling is controlled by the fluid pressure in the hydraulic cooling circuit, to provide a flow of cooling fluid to each of the clutches of the multiple friction transmission.
Often, the cooling system of a DCT limits the total oil flow to both clutches in the event that no hydraulic fluid or only a low amount of hydraulic oil is needed. Typically, a flow limiter is arranged as a differential pressure regulator keeping the pressure drop over the flow regulators constant. This feature is meant to limit the pressure in the cooling lines so that the low pressure pump can work at a lower pressure, and thus consumes less power. However, often the clutches require different cooling due to the difference in clutch power dissipation. Currently this is not optimally handled by the cooling strategies in the prior art, resulting in a reduced efficiency.
It can be challenging and/or complicated to control and regulate the hydraulic system for a multiple friction transmission to achieve the desired vehicle occupant comfort and safety goals. A proper timing and execution of events are required for efficient and/or smooth gear shifting.
So, there is a need for a hydraulic system for a multiple friction transmission that addresses at least one of the above mentioned drawbacks while maintaining the advantages.